Laying asphalt paving material on road surfaces entails spreading paving material consisting of an aggregate filled bituminous mixture on a prepared roadbed. The paving material is spread while hot and is then compacted so that a hardened pavement surface is formed upon cooling. Conventional paving machines utilize a heavy assembly termed a “screed” that is drawn behind the paving machine. The screed assembly includes a replaceable screed plate that is constructed of a suitable steel, to spread a smooth even layer of paving material on the prepared roadbed. The weight of the screed assembly aids in compressing the paving material and performing initial compaction of the paving material layer. Screed assemblies can include vibratory mechanisms placed directly on the screed plate or separate vibratory tamper bars connected in tandem with the screed plate to aid in the initial compaction of the paving material.
To facilitate laying of the paving material, the screed plate is typically heated, to a temperature in the range of about 82° to 171° C. (180° to 340° F.). Heating the screed plate assists the paving material in flowing under the screed plate and reduces adhesion of the paving material to the screed plate. If the screed plate is not adequately heated, the bituminous mixture contacting the bottom of the screed plate begins to harden, resulting in buildup of paving material and excessive drag.
Conventional screed plates are commonly heated by fossil fuel powered burners that heat the upper surface of the screed plate by the direct application of flame or hot exhaust gases. The use of fossil fuel burners to heat screed plates has several drawbacks. Combustion of fossil fuels generates smoke that represents a source of environmental pollution, and also poses a poor working environment for the paving workers. Additionally, because the flames or exhaust gases of the burners actually contact the screed surface, warping may result. The contour of the screed plate determines the quality, evenness and smoothness of the paving material that is being laid down. Screed plates are often flexed under extreme tensile loads during use to achieve desired crowning or other surface contours.
One alternate heating system that represents an improvement and which addresses the environmental drawbacks is disclosed in U.S. Pat. RE 36,981 issued Dec. 5, 2000 to Ralph Birtchet and assigned to Universal Screed Inc. Birtchet discloses the use of an elastomeric, electrically powered heating pad assembly positioned on the upper surface of the screed plate with a layer of insulation placed on top of the heating pad assembly. A heavy steel retainer plate assembly is disposed on top of the insulation to hold the heating pad assembly and the insulation in contact with the screed plate. But for the retainer plate assembly, the heating pad assembly and the layer of insulation are loosely positioned on the upper surface of the screed plate, mechanically constrained in their installed positions only by the forward face plate portion and the tail portion of the screed plate, and the side plates. The screed plate is attached directly to the deck plate of the paving machine.
Assembling the screed plate to the paving machine is generally a time consuming process because the screed plate must be precisely oriented relative to the frame. Assembly of the screed plate to the frame typically involves time consuming shimming operations. Inasmuch as the screed plate is a wear component, replacement due to wear may result in considerable downtime, often for days inasmuch as this shimming process must be repeated. Similar delays may result when the screed plate is removed to access the heating assembly for replacement or repair.